Question

how to calculate the wavelength frequency

Answer 1

actually
wavelength:-separation between consecutive crest or consecutive trough is known as wave length .
ex:-when given two consecutive crest separation is equal to 20cm then wavelength is also 20cm.

now
frequency:-number of oscillation per second is know as frequency .
ex:-if particle oscillating 100times in 4sec
then frequency=100/4=25sec^-1=25Hz

now if string wave produce then
relation between wavelength , frequency and speed of wave is
speed=frequency x wavelength